Fox News obtained the notebook in which Laundrie, 23, admitted killing Ms Petito, 22, and perversely claimed he thought her death was “merciful”.
“I ended her life," Laundrie wrote in a notebook recovered by the FBI from the Myakkahatchee Creek Environmental Park alongside Laundrie’s body on 20 October 2021.
“I thought it was merciful, that it is what she wanted, but I see now all the mistakes I made.
“I panicked. I was in shock. But from the moment I decided I took away her pain, I knew I couldn't go on without her.”
Laundrie died by suicide from a self-inflicted gunshot wound to the head in a reserve five miles from his parent’s home in North Port, Florida.
In the confession note, Laundrie said he decided he “couldn’t go on” after strangling his girlfriend of nearly three years.
“From the moment I decided, took away her pain, I knew I couldn’t go on without her.”
He also asked for sympathy for his parents.
“Please do not make life harder for my family, they lost a son and a daughter and a daughter – the most wonderful girl in the world. Gabby I’m sorry.”
A coroner ruled Ms Petito died by manual strangulation. Her body was found on the border of the Grand Teton National Park in Wyoming on 19 September after a month-long, nationwide, missing persons search that captured the attention of the country.
Laundrie claimed in the confession note that Ms Petito had been injured after falling into a creek prior to her death.
“I found her breathing heavily gasping my name, she was freezing cold,” he wrote.
“I carried her as far as I could down the stream towards the car, stumbling exhausted in shock, when my knees buckled and knew I couldn’t safely carry her. I started a fire and spooned her as close to the heat, she was so thin, had already been freezing too long. I couldn’t at the time realize that I should’ve started a fire first but I wanted her out of the cold back to the car.”
Laundrie wrote that his girlfriend’s feet and wrists hurt, and she was violently shaking from the cold.
He claimed she was “gasping in pain, begging for an end to her pain”.
“I don’t know the extent of Gabby’s injurys (sic). Only that she was in extreme pain.”
Ms Petito and Laundrie set off in their campervan on a cross-country trip in July 2021 and had been documenting their travels on YouTube.
On 1 September, Laundrie returned to his parents’ home alone.
Ms Petito’s parents reported her missing on 11 September after they couldn’t get in contact with her and Laundrie was refusing to say where she was.
Laundrie went on the run days later with his family telling police he was heading into the nature reserve close to their home.
On 19 September, Ms Petito’s remains were found in a Wyoming national park with her autopsy confirming that she had been strangled to death.
A month later on 20 October, Laundrie’s remains were found inside the Florida reserve, about five miles from his parent’s house.
His remains had been submerged in water for several weeks, but the note was preserved after being placed in a waterproof bag.
Attorneys for the Laundries and Petitos met with the FBI in Tampa, Florida, on Friday, in which items belonging to Brian and Gabby were returned to the families.
Laundrie family attorney Steve Bertolino said he was sharing the notebook with Fox News “as a matter of transparency”.
Ms Petito’s parents Joseph Petito and Nichole Schmidt have filed a lawsuit alleging that Chris and Roberta Laundrie not only knew their daughter was dead, but knew the location of her body, as they frantically searched hoping to find her alive.
They appeared for a preliminary hearing at the Sarasota County Circuit Court on Tuesday where a judge was deciding whether the case can proceed to trial.
The Petitos allege Brian’s parents released a misleading statement through their lawyer that their daughter was still alive when they knew their son Brian had already killed her.
They also claim the Laundries were helping Brian evade law enforcement.
The Laundries have sought to dismiss the lawsuit, and deny any knowledge of Ms Petito’s killing.